- Carbon Write-off is actually OK – Would you use?
My mate has been involved in an off with a vehicle. It pranged and dented the front wheel which will need replacing. His lbs wrote the bike off for insurance purposes and he received a replacement. Trek Madone 3.1.Posted 3 years ago
However, he works for GKN Aerospace and has had the forks and frame NDT (non-destructive tested). The ultrasonic feedback showed no damage at all. Should I use it after replacing wheel?cookeaaSubscriber
I’ll admit I’m no expert on NDT, but I was given to understand that composites are generally quite difficult to look at, especially if you don’t actually know the material thicknesses and layup to start with…
Don’t get me wrong, most of the people I have spoken to on the subject spend most of their time looking for defects in steel structures, so are not all that familiar with carbon themselves but seemed to reckon there would be a couple of ways to get “false positives” when checking the integrity of a composite body…
NDT covers quite a number of different techniques, specifically how was the frame “NDTed”?Posted 3 years ago
Yea I’m fairly sure that any NDT done on a composite is inaccurate and will give plenty of false readings, purely due to the fact that you don’t know the thickness of the laminates or the layup of the fibres. Even if you did know that, ultrasonic testing only gives you a signal feedback so it’s impossible to differentiate between a fibre-matrix boundary and a crack.
In short; I wouldn’t be riding it.Posted 3 years agojohanMember
Various kinds of NDT including ultrasonic is frequently used within the aerospace industry, if the guy that done the inspection knows what he is doing then he would find eventual cracks as he would be doing it daily on composite components for aircrafts.
Considering GKN produces composit components for various kind of aircrafts i would assume he knows very well what he is doing.Posted 3 years ago
ndt can be done on carbon fibre no problem, being ultrasonic testing, or dye pen. inspection.
UT being the one used, is the one ill go into a little more detail.
grain boundaries will simply show up as noise along the bottom of the display, a crack attacked at the correct direction (90 degrees)at the correct sensitivity should take your screen off, and confirmed using a swivel scan
to the untrained eye, all signals look the same, but all have their own little characteristics, for instance, a root crack, lack of root fusion and root over penetration on a weld prep can all be identified, even if they all show up within 3mm of each other on the display, same principle with carbon fibre.
you do not need to know any depths, acoustic velocities or acoustic impedance for finding defects, only for locating and sizing them for reports etc.
craig – PCN number 311102.Posted 3 years ago
craighill, you seem to be describing an angle beam scan for weld inspection, CFC has no grain boundaries and typically suffers from delaminations which require a normal incidence scan to detect. I very much doubt that this frame has been 100% ultrasonically inspected, this would require 10 of hours of meticulous hand scanning with significant prior knowledge of the layup.Posted 3 years ago
kane, I was just using the weld inspection as an example in order to prove signal feedback can be interpreted as defects or false indications.
in my opinion not 10hours, I can do a full scan compression and shear on a 1.5tonne tension load ring housing for shell mars b project in less than that.
I am however guessing, coming from a aerospace background, the operators know about scanning compositesPosted 3 years ago
Composite by its very nature is not a homogeneous isotropic material. Most inspections of a metallic structure only require an inspection of specific areas of the assembly such as welds, there is generally no requirement to ensure that the bulk of the material is free from defects. To prove a composite structure is free from defects it is necessary to inspect the entire area, metals tend not to suffer from barely visible impact damage. Also, how do you distinguish a ply drop off from a near back wall delamination without prior knowledge of the layup.Posted 3 years agosuburbanreubenMember
So why are the Insurance Co allowing a written off frame to remain in circulation? To all intents and purposes it may be fubarred yet could be sold on to some innocent purchaser who could do themselves a serious mischief. This one appears to be OK but how many aren’t?Posted 3 years ago
I disagree, many, infact 90% of my jobs I do on metals require 100% ultrasonic examination, some with different spec’s for weld ends, others just require weld ends, just depends on customer, application etc.
I am unable to answer regarding your last question kane, I have not done nearly enough testing of composites, however I assume, no I hope, the guys working on aerospace composites could test a composite material and happily put their signature on it.Posted 3 years ago
It is of course impossible to distinguish between a ply drop off and a delamination near the back surface, the impedance change at a delamination is exactly the same as a change in thickness. I’ve had a go at inspecting several composite bicycle frames, from the very first that appeared on the scene to some of the latest models. This is composite for the consumer market, and best described as variable, both in volume fraction and porosity levels. Both these variables plus the additional secondary bonding processes make any inspection very challenging.
I stand corrected on the inspection of metallic structures, I assumed that most stock that was purchased for critical applications was inspected by the manufacture, that’s certainly my experience within the aerospace industry, the exception being forgings which had a secondary inspection for laps and inclusions. Out of interest, when you perform a 100% inspection of a metallic part what are you looking for and what would be the typical minimum detectable defect size.Posted 3 years ago
It varies massively from part to part, I work mainly within the oil&gas and nuclear, the two are treated very differently with regards to procedures and specifications, generally within castings your looking for shrinkage, porosity, laps, cracks&tears, inclusions, aswell as testing welds etc.
Minimum detectable size can be down to a single gas pore, it comes down to grain structure, material etc, and type of defect. Shrinkage doesn’t give a clean signal, tends to reflect it off all over the place, your looking more for a loss in backwall generally 50%, whereas a crack will go off the screen. so there isn’t really a minimum detectable defect, it varies too much. With regards to specs, it varies massively again, depending on job the things can be tested 2 or 3 times on ut and 6-8 times for mpi, 100% initial, test of excavations, test of repairs, 100% test after post weld heat treatment , 100% test after machining, and then a witnessed test by customer, it all depends in the job in hand, a good example is mpi, one specification states 1.6mm linears to be considered relevant and are to be repaired, to the next job being anything under 13mm linear is acceptable.Posted 3 years ago
The insurance company told him to chuck it away. I’ve been using it as a training bike on a turbo due to the crap weather we’ve been having. No sale. Now thinking about taking it out in crappy weather as johnnystorm has suggested. Interesting comments from those that have industry knowledge, thanks.Posted 3 years ago
I’m pretty confident about his ability as an engineer and he does indeed test carbon structures on a daily basis. With this in mind and the advice above I shall venture out into the big wide world!thisisnotaspoonMember
I’d think it would be fine. Seeing as most roadies I know don’t bat an eyelid riding a repaired carbon frame, and a repaired frame must have had more strain than one with no visible damage (if you remove one of the tubes the rest of the frame is goingt o bend a lot!).Posted 3 years ago
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